Re: gigantism as liability

[Augusto Haro <augustoharo@gmail.com>]

I read somewhere that the fact there is not yet evidence of some
morphotypes (in this case, terrestrial mammalian giants competing in
size with sauropods) does not mean there is a constraint: so to say,
with the same logic in the Eocene we (or some hypothetical human
transplanted there without knowing the Holocene fauna) should have
conclude that the wolf morphotype cannot exist.

But is there a greater tendency to proportionate size increase in
dinosaurs than in mammals? After all, when comparing dinosaurs to
mammals, there seem to be size increase from quite small (rat-like
sized, I grossly suppose) therian ancestors, while in dinosaurs the
size increase was from a relatively larger original creature, somewhat
of the rooster-sized Eoraptor or Heterodontosaurus. This may depend on
the taxonomic level we take for comparison, granted.

As for the size increase in sauropods, the basal state for Sauropoda
should have been Vulcanodon, an already large animal, larger
apparently than the earliest elephants or rhinos. When comparing size
increase in two clades, I think we should get the tree of both
monophyletic groups to be compared, then recognize the most
parsimonious assignment of relative size change per branch, on all
branches of the phylogeny, and then see if the mean relative increase
in size per branch in one taxon superates that of the other. We can
take into account OTU diversity to discount its effects (the largest
the diversity, the smaller will likely be the mean transformations in
relative size), and the relative size increase relative to absolute
time.

After all, if you have some taxon evolving for a longer time span, it
would likely reach greater size by having more time of evolving in
that direction (accepting there is a trend promoted by natural
selection, but the same would apply to the taxon sorting in
macroevolutionary models: the more the time of a lineage evolving, the
most disparate tend to be the end-results). I do not say with this
that we can explain the lesser size of placental mammals because of
the Cenozoic being shorter than the Jurassic+Cretaceous, as the
largest sauropods may be Jurassic, just to consider that the time span
involved by a monophyletic taxon should be taken into account when
making these comparisons.

Now, accepting there is really a size constraint on mammals not
present in dinosaurs, somewhere I read that the Carboniferous
arthropods can grew so big because of the oxygen was much more
concentrated and easier to diffund in their non-closed circulatory
system. Perhaps, and accepting with Wedel that the basal state for
Saurischia at least, and perhaps for Ornithodira as a whole, was
having an enhanced system for oxygen uptake, something similar can
apply. However, this may not be so good an explanation as the sea
arthropods, past and present, grew larger, while oxygen concentration
is always lesser in the water, and as the whales grew apparently
larger than most dinosaurs -completely known ones at least- with a
less efficient oxygen-uptaking system.

If the adaptations for gravity-resistance are more important than the
oxygen uptaking, perhaps the saurischians with air sacs are just too
much "filled of air", with much of their visceral cavity (and not just
bones) filled in air, so that they may be, for a given external
volume, be much lighter than a mammal of the same external volume, as
when comparing a chick to a mammal of similar volume. Thus, a sauropod
weighting the same of Baluchitherium should have been more voluminous
at sight.

Perhaps there is no more explanation for the fact that the largest
dinosaurs are largest than largest terrestrial mammals, than for what
reason the largest cetaceans are larger than the largest plesiosaurs
(here the gravity arguments should not apply so easily, I suppose), or
why the largest ichthyosaur known is larger than the largest
elasmosauroid, or why the largest misticetes are larger than the
largest odontocetes, etc. Why should both taxa in a comparison show
the same maximum size? It should be a great coincidence, the most
expected thing would be that one presents a maximal size greater than
the other.